The objective of this project was to validate the elimination of acid cleaning of high temperature salt water heat exchangers onboard Navy ships. High temperature heat exchangers scale heavily as a result of operating at wall temperatures above 150°F. Scaling is a physical phenomenon resulting from the reduced solubility of calcareous salts at wall temperatures above 150°F. Hot inlet gas can cause a large excursion in the tube-wall temperature on the seawater side of the heat exchanger. Dissolved solids are precipitated in the seawater coolant, thus forming calcareous deposits on the tube walls. These deposits corrode and erode the walls causing cracks and holes. The corrosion and erosion resulting from scaling usually cause the heat exchanger failure. As a result, in-situ and shore-based depot chemical cleaning, both costly and man-power intensive, are required to help prevent corrosion and erosion, which lead to cooler failure. These processes use various cleaning chemicals, such as tri-sodium phosphate, sulfamic acid, and sodium carbonate. To support these cleaning procedures, the shore-based activities are required to carry excess hazardous materials, which can create up to 10,000 gallons of hazardous waste per application, with a disposal cost of $2.58/gallon.
To eliminate scaling and the use of hazardous chemicals in bleed air system heat exchangers, a heat pipe bleed air cooler (HP-BAC) heat exchanger configuration was substituted for an existing shell-and-tube configuration. Heat pipes provide a controlled intermediate fluid of water for the transfer of heat between the high temperature bleed air and low temperature salt water. The ability to closely control the temperature of water in a hermetically sealed pipe makes it possible to maintain the wall temperature at the cold end of the heat pipe below 150°F. Within the heat pipes, water evaporates after being heated by the hot air at the bottom of the pipe. The water then condenses at the top of the pipe, which is cooled by the seawater. The heat transfer surface temperature on the condensation side is controlled by the ratio of the surface areas on the hot and cold sides of the pipe.
In the land-based testing, tube wall temperatures remained below 150°F. The actual reduction in fouling, hazardous material reductions, and the required maintenance will be revealed by future shipboard testing and determine whether the HP-BAC should be moved to a production phase. An analytic model has been validated by the test results and benchmarked to the test data. It can be used to design a larger HP-BAC that will provide a one-to-one performance replacement for the shell-and-tube BAC.
Shipboard testing of an expanded full-scale prototype will aim to demonstrate that the HP-BAC significantly reduces the salt water scaling and its associated maintenance and environmental costs. The present fabrication methods are impractical and uneconomical for a production HP-BAC; however, potential cost improvements have been identified. These improvements would need to be considered when making a production decision following successful shipboard tests. The primary focus for transition would be new ship construction along with other commercial applications.